1. Field
The present disclosure relates to digital non-linear loop control circuits.
2. Background
FIG. 1 is a block diagram illustrating a system 100 with an analog feedback control circuit. The system 100 includes a parameter control circuit 110, a system processing circuit 120, and a parameter measurement circuit 130. The analog feedback control circuit comprises a resistor R, a capacitor C, and an operational amplifier 152.
The analog feedback control circuit is typically used to achieve a desired parameter value at the output. For example, the analog feedback control circuit may be used to achieve a desired output power. In this example, the parameter control circuit 110 may comprise a variable attenuator, and the system processing circuit 120 may comprise a power amplifier (PA). The variable attenuator is configured to attenuate the input signal by an amount that is controlled by a control signal from the feedback control circuit. In operation, the feedback control circuit measures the output power using the parameter measurement circuit 130, and adjusts the control signal to the attenuator in a direction reduces the difference between the desired output power and the measured output power. For example, if the measured output power is below the desired output power, then the feedback control circuit reduces the attenuation by the variable attentuator to raise the output power closer to the desired output power.
The characteristics and behavior of the analog feedback control circuit are governed by a set of equations that are well known in the art. One design constraint for the analog feedback control circuit is that the loop gain must be less than one for response frequencies for which the phase shift is greater than 180°. The well-known Bode plot is one graphical method of showing this constraint. In a linear system, it is well known that it takes approximately three time constants for the system 100 to settle to a value close to the final output value. Further, the time constant is typically about 10 times the inherent delay in the feedback circuitry.
The operation of the analog feedback loop may be been transferred to digital circuitry operating with the same basic approach of integrating the feedback in order to develop the control signal. However, this digital implementation has the same performance limitations as the analog version.